While there exists detection, treatment, and prevention techniques, dental caries remains a prevalent condition affecting people of all age groups. If not properly and promptly treated, caries could lead to permanent tooth damage and even to loss of teeth.
Traditional methods for caries detection include visual examination and tactile probing with a sharp dental explorer device, often assisted by radiographic (x-ray) imaging. Detection using these methods can be somewhat subjective, varying in accuracy due to many factors, including practitioner expertise, location of the infected site, extent of infection, viewing conditions, accuracy of x-ray equipment and processing, and other factors. There are also hazards associated with conventional detection techniques, including the risk of damaging weakened teeth and spreading infection with tactile methods as well as exposure to x-ray radiation. By the time caries is evident under visual and tactile examination, the disease is generally in an advanced stage, requiring a filling and, if not timely treated, possibly leading to tooth loss.
In response to the need for improved caries detection methods, there has been interest in improved imaging techniques that do not employ x-rays. One method that has been commercialized employs fluorescence, caused when teeth are illuminated with high intensity blue light. This technique, termed quantitative light-induced fluorescence (QLF), operates on the principle that sound, healthy tooth enamel yields a higher intensity of fluorescence under excitation from some wavelengths than does de-mineralized enamel that has been damaged by caries infection. The strong correlation between mineral loss and loss of fluorescence for blue light excitation is then used to identify and assess carious areas of the tooth. A different relationship has been found for red light excitation, a region of the spectrum for which bacteria and bacterial by-products in carious regions absorb and fluoresce more pronouncedly than do healthy areas.
Some proposed solutions for optical detection of caries have been discussed in patent literature.
U.S. Pat. No. 4,290,433 (Alfano) describes a method to detect caries by comparing the excited luminescence in two wavelengths.
U.S. Pat. No. 4,479,499 (Alfano) describes a method to detect caries by comparing the intensity of the light scattered at two different wavelengths.
U.S. Pat. No. 4,515,476 (Ingmar) describes use of a laser for providing excitation energy that generates fluorescence at some other wavelength for locating carious areas.
U.S. Pat. No. 6,231,338 (de Josselin de Jong et al.) describes an imaging apparatus for identifying dental caries using fluorescence detection.
U.S. Patent Application No. 2004/0240716 (de Josselin de Jong et al.) describes methods for improved image analysis for images obtained from fluorescing tissue.
Among commercialized products for dental imaging using fluorescence behavior is the QLF Clinical System from Inspektor Research Systems BV, Amsterdam, The Netherlands. Using a different approach, the Diagnodent Laser Caries Detection Aid from KaVo Dental GmbH, Biberach, Germany, detects caries activity monitoring the intensity of fluorescence of bacterial by-products under illumination from red light.
U.S. Patent Application Publication 2005/0003323 (Katsuda et al.) describes a hand-held imaging apparatus suitable for medical or dental applications, using fluorescence imaging.
U.S. Patent Application Publication 2004/0202356 (Stookey et al.) describes mathematical processing of spectral changes in fluorescence in order to detect caries in different stages with improved accuracy.
While the disclosed methods and apparatus show promise in providing non-invasive, non-ionizing imaging methods for caries detection, there is still room for improvement. One recognized drawback with existing techniques that employ fluorescence imaging relates to image contrast. The image provided by fluorescence generation techniques such as QLF can be difficult to assess due to relatively poor contrast between healthy and infected areas. As noted in the '2356 Stookey et al. disclosure, spectral and intensity changes for incipient caries can be very slight, making it difficult to differentiate non-diseased tooth surface irregularities from incipient caries.
It has been recognized that, with fluorescence techniques, the image contrast that is obtained corresponds to the severity of the condition. Accurate identification of caries using these techniques often requires that the condition be at a more advanced stage, beyond incipient or early caries, because the difference in fluorescence between carious and sound tooth structure is very small for caries at an early stage. In such cases, detection accuracy using fluorescence techniques may not show marked improvement over conventional methods. Because of this shortcoming, the use of fluorescence effects appears to have some practical limits that prevent accurate diagnosis of incipient caries. As a result, a caries condition may continue undetected until it is more serious, requiring a filling, for example.
Detection of caries at early stages is of particular interest for preventive dentistry. However, early caries can be undetected, so that by the time positive detection is obtained, the opportunity for reversal using low-cost preventive measures can be lost.
U.S. Pat. No. 6,522,407 (Everett et al.) describes the application of polarimetry principles to dental imaging.
It would be advantageous to provide a solution for caries imaging not concerned with measuring a degree of depolarization, preferably using a smaller number of components and not requiring switchable orientation of a polarizer between one of two positions.
In spite of some advances in imaging optics and the use of polarizers in the illumination and imaging paths, however, specular reflection remains a problem. This problem is particularly pronounced for illumination light at non-zero incidence to the polarizer, where light leakage from the crossed polarizers increases with increases in incident angle.
Thus, Applicants have recognized a need for improved non-invasive, non-ionizing imaging methods for caries detection that reduce specular reflection for improved accuracy for detection of caries, particularly in its earlier stages.